email_mirror: Filter out null characters in email bodies.

They're rarely useful, usually displayed invisibly in most tools
anyway, and this helps make sure the message makes it into Zulip
rather than being rejected.

zerver/lib/email_mirror.py

@@ -148,6 +148,8 @@ def mark_missed_message_address_as_used(address):
 def construct_zulip_body(message, realm):
     # type: (message.Message, Realm) -> Text
     body = extract_body(message)
+    # Remove null characters, since Zulip will reject
+    body = body.replace("\x00", "")
     body = filter_footer(body)
     body += extract_and_upload_attachments(message, realm)
     if not body: